1. Field of the Invention
The present invention relates generally to a system and method for minimizing new water use in the circulation system of a treatment plant.
2. Background of the Invention
Many purification and separation processes are performed wet, in other words with the introduction of water to function as the carrier of pollutants and interfering substances as well as for the recyclable fraction. If no other measures are taken, the concentration of pollutants and interfering substances quickly rises rendering, for example, purification processes ineffective. Pollutants and interfering substances can also impair the sequence of operations during purification or separation or during subsequent treatment operations. Therefore, it is necessary to limit the amount of pollutants and interfering substances as much as possible. It is to this end that new water is introduced into the water circulation system.
A regulating process for minimizing water use in a water circulation system of a paper, cellulose or wood plant is disclosed in WO 99/01612. Here, the concentration of interfering substances is regulated either in a paper machine circuit and/or in a filtering circuit, preferably in the final filtering circuit, whereupon the interfering substance removal and also the new water input are regulated. Sensors that record certain parameters that measure the interfering substance concentration have been placed in suitable locations in the water circulation system. This includes the assessment of, in particular, turbidity and cationic requirements.
The use of sensors should be avoided since they are susceptible to interference.
It is an object of the present invention to provide a process of the type mentioned above, which allows minimizing new water use without unduly letting the interfering substance concentration rise.
The invention provides a method for minimizing new water use in the water circulation system of a treatment plant in which,
a) the matter to be treated is purified and/or disaggregated in a treatment step using water separating the matter to be treated into different components that are not necessarily of the same variety, at least one of which components is removed from the treatment step;
b) the suspension containing the remaining components is subjected to mechanical purification whereby solid parts with dimensions that exceed a specific threshold value are removed from the suspension;
c) the mechanically purified suspension is separated into a first processing water stream and a second processing water stream;
c-1) whereby the first processing water stream is led back to the treatment step; and
c-2) the second processing water stream is subjected to a chemical-physical clarification;
d) the chemically-physically clarified processing water stream is separated into a first clarified water stream and a second clarified water stream,
d-1) whereby the first clarified water stream is led into the first and/or the second processing water stream and/or into the suspension and
d-2) subjecting the second clarified water stream to a biological clarification; and
e) the clarified water stream that was biologically clarified, being a fresh water stream, is led into the first and/or second clarified water stream, whereby the ratio of the first processing water stream to the second processing water stream and of the first clarified water stream to the second clarified water stream has been pre-determined depending on the matter that is supplied to the treatment step and on the type of mechanical purification and on the type of chemical-physical clarification used and the water circulation system is basically closed so new water is introduced only when the concentration of dissolved organic and inorganic particles exceeds a specific threshold value.
The invention takes advantage of the fact that the composition of the waste to be treated is known and remains reasonably constant so that certain pollutants and interfering substances effectively can be removed from the water circulation system avoiding constant new water requirement. This is particularly the case with waste similar to waste collected from the Gelben Sack [yellow bag] or the Gelben Tonne [yellow barrel] container, which is collected in Germany by the Dualen System and which is regularly pre-sorted before being subjected to wet separation. Wet separation is generally used for light packaging waste such as plastics, aluminums, cardboard foil composites, paper composites and other composite materials that still contain pollutants and interfering substances when they have been treated, for example, in accordance with the method described in WO 98/18607 in such a manner that metallic substances and certain plastics no longer are suitable for wet separation. In the clarification steps according to embodiments of the invention, pollutants and interfering substances can be effectively removed from the water circulation system. It has been shown that it is not necessary to continually examine the fresh water but that examination at longer but regular intervals, approximately every two weeks, suffice to identify a possible increase in concentration. Purification and separation processes remain consistent since water purification, according to embodiments of the method of the invention, also can be adjusted to treat more polluted materials.
Paper separation plays an essential role when treating light packaging waste and simply completely, if possible, removing paper fibers from the water circulation system can purify the water.
Additionally, an implementation of the method allows the following steps to occur during mechanical purification after step b) either alone or in combination:
b-1) Sieving of the suspension; preferably using a sieve having a mesh size of 2 to 6 mm and preferably having a mesh size of 4 mm. Sieving facilitates segregation of coarse organic pollutants such as plastic fragments.
b-2) Leading of the suspension through a hydro cyclone containing the heavy matter in the underflow and all other components in the overflow. In the case of light packaging waste, the suspension would contain mainly paper fibers whereby inorganic heavy pollutants are removed from the hydro cyclone in the underflow. The overflow still contains the paper fibers as well as organic fine pollutants.
b-3) Filtration of the suspension preferably using a filter having an approximate mesh size of 150 xcexcm. The mesh size is then sized to fit the size of the parts that have to be withheld. The indicated size effectively separates the paper fibers. The paper fibers remain on the filter and can be used later for example in a paper recycling plant.
Another implementation of the method allows the implementation of the following steps either alone or in combination during the chemical-physical clarification according to step c-2):
c-2-1) addition of water purification chemicals in one and/or two steps, for example, precipitation agents and/or flocculants. Subsequent doses of, for example, cationic-actives and anion-actives could be added. Dual flocculation is recommended if high purity of the liquid phase to be separated is required.
c-2-2) separation of the flocculated pollutants from the purified water using flotation and/or sedimentation and removal of floating solids or deposited sediment or removal of the clarified water located in between them.
Biological clarification usually occurs in municipal sewage works.
Water treatment remains in the forefront of the method according to embodiments of the invention, not the production of paper fibers.